Catalysts are employed in the exhaust systems of automotive vehicles to convert carbon monoxide, hydrocarbons, and nitrogen oxides (NOx) produced during engine operation into nonpolluting gases. When the gasoline powered engine is operated in a stoichiometric or slightly rich air/fuel ratio, i.e., between about 14.7 and 14.4, catalysts containing precious metals like palladium and rhodium are able to efficiently convert all three gases simultaneously. Hence, such catalysts are often called "three-way" catalysts.
Diesel engines are commonly used in Europe and they are able to obtain enhanced fuel economy. Such engines operate under oxygen-rich conditions where the A/F ratio is greater than 19, generally 19-40. Three-way catalysts are able to convert carbon monoxide and hydrocarbons in such lean-burn (excess oxygen) operation but are not efficient in the reduction of NOx (NO+NO.sub.2) during diesel operation. Efforts have thus been made in developing catalysts able to remove NOx under net oxidizing conditions in recent years. Development of efficient diesel catalysts to meet European upcoming diesel emission standards continues to prompt research into such development. Due to the nature of diesel engine exhaust, these catalysts must be able to reduce NOx at relatively low temperatures in an oxygen rich environment.
Diesel catalysts are often based on zeolite materials containing a precious metal like platinum which can have major drawbacks. Among the most important are a narrow temperature range of operation and loss of activity (and sometimes physical integrity) under the hydrothermal conditions of automotive exhaust gases. For example, a carrying platinum is generally only active at a relatively low temperature, i.e., less than 250.degree. C. At higher temperatures the competitive oxidation of the reductant hydrocarbon molecules by oxygen is so fast that the removal of NOx drops off precipitously with rising temperature so as to make such catalyst inadequate for treating somewhat hotter exhaust streams. Conversely, when the active sites are transition metal ions exchanged into the cationic sites of the zeolite, the onset of SCR activity begins at temperatures greater than 400.degree. C., which renders the catalyst inactive for catalysis during a large portion of the necessary temperature range of the desired catalytic operation. Other diesel catalysts are based on support materials such as silica, gamma-alumina, titanium oxide, zirconium oxide or some combination thereof. These catalysts have the drawback, however, that they either have rather low NOx conversion efficiencies or have narrow NOx conversion temperature windows.
Platinum impregnated alumina materials are considered viable candidates for the after-treatment of diesel exhaust at low temperatures. Such catalysts act to reduce the NOx through the use of hydrocarbons over a catalyst, the hydrocarbons being in turn oxidized. Previous work Kintaichi, Y.; Hamada, H.; Tabata, M., Sasaki, M., Ito, T., Catalysis Letters, 6 (1990) 239 has shown that modification of alumina with silica commercial leads to loss in the catalytic efficiency of alumina for decomposition of NO into N.sub.2 in the presence of propane. Propane is commonly used as a test gas in simulated diesel exhaust gases. Furthermore, impregnation of alumina or alumina-silica with platinum leads to reduced conversion temperature and comparable efficiency for both systems. In a related study, Burch, R, Millington, P. J., Catal. Today, 29 (1996) 37 observed the activity of platinum-alumina and platinum-silica for NO conversion with propene and found that platinum-silica was more active than platinum-alumina. The contrasts of these results have been explained on the basis of the reactivity of platinum with various gases with the supports playing a minor role by modifying the metal surface properties. These studies suggested to us that platinum on alumina-silica would be less effective than platinum-alumina or platinum-silica depending on the choice of hydrocarbon as reductant.
We have now unexpectedly found that the NOx conversion efficiency of platinum-alumina can be improved significantly when a catalyst is made of platinum on silica/alumina if a particular composite oxide of silica/alumina is made by sol-gel techniques. The amount of silica in the composite oxide support is critical since improved NOx conversion activity is obtained upon increasing silica content from 10 to 30% by weight while further increase in silica content to 50% by weight results in a decrease in NOx conversion. Our explanation of this will be explained in detail below.